At the other extreme, some birds have jettisoned hind limb function
for particular selective flight advantages; because the hind limbs
are the primary launch power in birds, these species often need
gravity assistance to launch. Such taxa (like Phaethon, Fregata,
Apus, and Chordeiles) are clumsy on the ground but this has nothing
to do with scaling (in fact, Apus and Chordeiles are quite small).

Good point, and let's not forget hummingbirds, the smallest birds of
all. Do they ever walk at all?